you do not need to configure any files in /etc.
The issue at first glance seems to that you may not have a video source created, since the capture works with other programs.
1. confirm in mythtv-setup that under video sources you have created a source.
2. under input connections select input you want to attach video source to
3. give source a name and select source from drop down menu.
take a look at http://www.mythtv.org/wiki/index.php/Mythtv-setup
the way mythtv is set up is no different then if mythtv was a installed by itself.
I have been using mythtv for years and I have never edited a file in the /etc directory. I have also been using Plutohome for a while and I also have not had to edit any files in the /etc for MythTV.